As can be seen in the recent worldwide efforts directed toward nanotechnology, as a trend in the materials development, there can be mentioned one which is focused on a smaller structural unit and aims at controlling the structure. In such a technical trend, the present inventors have conducted researches on the sheet forming technology of microfibrilated cellulose (hereinafter abbreviated as MFC) obtained by using as a raw material a natural cellulose such as a pulp which contains an abundance of fibers having a thickness of 1 μm or less as disclosed in Patent Document 1 and Non-Patent Document 1 and that of a fine and highly crystalline cellulose nanofiber having a fiber diameter in a range of about several nm to 200 nm produced by acetic acid bacteria disclosed in Patent Document 2 (bacterial cellulose, hereinafter abbreviated as BC).
It has been known that a sheet comprising cellulose fibers having such minute fiber diameters has very unique material characteristics such as an extremely high mechanical strength as described in Non-Patent Document 2 and at the same time an extremely low linear expansion coefficient as disclosed in Patent Document 3 and Patent Document 4. It is described in Patent Document 3 and Patent Document 4 that a hybridized material in which spaces therebetween are filled with a resin exhibits a low linear expansion coefficient.
In addition, Patent Document 5 has recently disclosed that a hybrid film in which an epoxy resin or an acrylic resin is hybridized with a BC sheet obtained by compressing and then drying a BC gel obtained by static culture has a low linear expansion coefficient as well as high transparency, and therefore it is effective as an optical film and an optical substrate.
However, according to Patent Document 5, for example, since the BC sheet from static culture is a sheet having a very dense structure, it takes an extremely long time in the hybridization step to impregnate a cellulose fiber sheet having a porosity of about 30% with a resin monomer, which requires impregnation under reduced or increased pressure (for example, by carrying out impregnation for 12 hours under a reduced pressure of 0.08 MPa), and therefore it has been disadvantageous from a viewpoint of industrial production.
Besides, the cellulose fiber sheet mainly contributing to the realization of low linear expansion coefficient in the art disclosed by the document is intrinsically a hygroscopic material, and, accordingly it has a property to easily cause changes in physical properties when absorbing moisture. Therefore, there was a demand to reduce the fraction of BC or cellulose as much as possible in a hybrid film to a level which could maintain the feature of low linear expansion coefficient. In other words, this is a demand to increase the ratio of volume occupied by pores (porosity) to which a resin can impregnate in a cellulose nonwoven fabric.
In the meantime, as is disclosed in Patent Document 6 and Patent Document 7, the cellulose nonwoven fabrics comprising nanofibers as described above can be expected to function as a separator in an electric storage device and have high filtering properties such as those of a HEPA filter due to the fine network structure made by nanofibers. In such a field, nonwoven fabrics are required to have filter performance to intercept minute substances as well as high air permeability. In order to satisfy these requirements at the same time, a technology to control the pores of the sheet to a minute size while increasing the porosity has been needed. A technology to form a nonwoven sheet with fibers of a minute fiber diameter such as nanofibers was paid attention in Patent Document 6 and Patent Document 7 as the measures to solve the requirements.
From the viewpoint mentioned above, a technology to provide a nonwoven fabric comprising BC or fine cellulose fibers which can be a material of a matrix of hybrid film, a separator or a functional filter is expected, and the use of a static culture sheet of BC can be considered as one of the solutions thereof as described in Patent Document 5. However, when the nonwoven fabric is produced, it is desirable to conduct the production not by a batch process but by a continuous production process both from the viewpoint of industrial productivity and from the viewpoint of degree of freedom of applications of the product and therefore, the use of the static culture sheet of BC was in disadvantageous situation particularly because from the viewpoint of production process. This is because when the static culture sheets of BC are continuously produced, there arises a problem in the point of productivity due to the slow sheet forming rate (it usually needs time period of around 5 days to form sheets from a static culture gel having a sheet thickness of around 1 cm), and because there is no technology to continuously produce such a slowly produced sheet while controlling high quality. A technology to form sheets from nanofibers by an industrially applicable artificial process has been demanded.
As a technology to industrially produce nonwoven fabrics comprising nanofibers such as BC, Patent Document 7 and Patent Document 8 describe Examples of forming sheets from BC by a paper making process which is a production process of paper. Furthermore, there is disclosed in Patent Document 9 a paper making sheet forming technology of MFC corresponding to subnanofibers obtained by making fine wood pulp.
However, although these documents describe sheet forming processes of BC and MFC, they mainly emphasize only on a drying method as an important point to pay attention as compared to the case where fibers derived from pulp used for normal paper making are used and do not describe any particular point to take notice for the other steps. In fact, when relatively thin nonwoven fabrics suitable, for example, for a separator of electric storage devices are attempted to produce based on the information disclosed in these documents by performing paper making of nanofibers such as BC, it was difficult to produce high quality nonwoven fabrics having no pinhole and high uniformity stability and with high efficiency using an existing paper machine.
The uniformity referred to here means whether the distribution of sheet thickness is uniform or not at a resolution level of several mm to 10 mm at least for the sheet surface. For example, when the nonwoven fabric is used as a separator for electric storage devices mentioned above, relatively thin films having a size of about several mm as the minimum value of a slit width (in the case of tape-like form) or a diameter (in the case of a circular film) and a sheet thickness of 60 μm or less are often used. For such a use, homogeneity in physical properties (strength, air permeability, etc.) of a film by this size unit is required. Particularly when a film is a thin nonwoven fabric, the homogeneity in physical properties of the film can be approximately expressed as equivalent to the uniformity of sheet thickness. In addition, when the nonwoven fabric is used as the base material of an optical substrate mentioned above, it is natural that the optical uniformity of the substrate sheet surface is required at a high level, and for that purpose high sheet uniformity of the nonwoven fabric which was a base material was demanded.
That is, it has been necessary to solve some problems mentioned above to enable to commercially provide a nonwoven fabric comprising fine cellulose fibers as a material having the high function described above with a quality applicable to the fields in which the characteristics thereof are made good use of.
Patent Document 1: JP-A-56-100801
Patent Document 2: JP-B-6-43443
Patent Document 3: WO03/040189
Patent Document 4: JP-A-2004-270064
Patent Document 5: JP-A-2005-60680
Patent Document 6: JP-A-9-129509
Patent Document 7: J P-A-2004-204380
Patent Document 8: JP-A-10-125560
Patent Document 9: JP-A-10-140493
Non-Patent Document 1: J. Appl. Polym. Sci. Appl. Polym. Symp. 37, 797-813 (1983)
Non-Patent Document 2: 13th Polymer Materials Forum Preprints, pp. 19-20(2004)